The present invention relates to a sea sled for entrenching and burying undersea pipeline and particularly relates to an eductor system for removing slurry from the trench whereby pipeline laid along the sea bottom settles into the trench.
Various systems for laying pipelines along the sea bottom have been proposed and utilized in the past, (see, for example, U.S. Pat. No. 3,751,927). Certain of these systems provide a sea sled having jet nozzles directed to fluidize the sea bottom and form a trench for receiving the pipeline. Air jet type eductor systems have also been provided for removing the cuttings or slurry formed by the jet nozzles from the trench. Such air jet eductor systems usually comprise a nozzle disposed in the inlet end of a discharge conduit with air being supplied under pressure from a surface floating tender. Air jet eductor systems are very efficient and will lift significant quantities of slurry. However, with the increasing necessity to lay pipelines in deeper water, for example in water depths exceeding 150-200 feet, an air jet eductor system becomes uneconomical. That is, as the water depth increases, the horsepower requirements to supply compressed air from the surface increase quite rapidly.
U.S. Pat. No. 3,877,238, issued Apr. 15, 1975 and commonly assigned with the present invention, describes an improved sea sled for entrenching and pipe burying operations which has been found to successfully overcome problems associated with prior art devices in deep water operations. The present invention constitutes an improvement upon the sea sled and eductor system described in the U.S. Pat. No. 3,877,238 in that it better utilizes energy available from the high pressure, low volume fluid source pumped from a surface floating vessel and in that slurry is discharged from the eductor pipes in such a way as to more effectively avoid filling the trench with spoil before the pipe has a chance to settle.
The present invention provides a novel and improved sea sled and eductor system for forming a trench and for burying underwater pipelines, particularly in depths beyond the feasible or practical limits of presently available air operated equipment, which minimizes or eliminates problems associated with prior sea sled and eductor systems and provides a novel and improved sea sled and eductor system having various advantages in construction, mode of operation and use in comparison with such prior sea sleds and eductor systems. In considering the problem of entrenching and burying pipelines in deeper water, for example, in water depths beyond 200 feet, a water jet eductor system was proposed to overcome the problems associated with prior air jet systems and also in order to utilize an available low volume high pressure supply for the eductor. Upon further consideration, however, it was found that utilization of a high pressure water jet would give rise to severe cavitation problems in the throat and mixing region of the eductor conduit. In short, while a high pressure water jet could be utilized, the eductor system per se would have an extremely shoft life as cavitation effects would destroy the efficiency of the system as well as the equipment itself. Reduced flow rates and pressures from the high pressure water jet were rejected as solutions to the cavitation problem for a number of reasons including the desirability of obtaining a significant predetermined flow rate of slurry removal from the trench.
Accordingly, it is a primary object of the present invention to provide a novel and improved sea sled and eductor system for entrenching and burying underwater pipelines.
It is another object of the present invention to provide a novel and improved sea sled and eductor system for entrenching and burying underwater pipelines in deep water, for example, on the order of 200 feet or more.
It is still another object of the present invention to provide a novel and improved eductor system for burying subsea pipelines.
It is a further object of the present invention to provide a novel and improved sea sled and eductor system for entrenching and burying subsea pipelines utilizing a novel and improved two stage jet pump configuration for suctioning the slurry from the trench in which the pipeline is to be laid.
It is still a further object of the present invention to provide a nozzle and improved sea sled and eductor system for entrenching and burying subsea pipelines in which slurry is discharged in a direction substantially parallel to the sea bottom to decrease the settling time and increase the distance which slurry is cast from the excavation, to thereby minimize the transport of spoil back to the trench.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.